That can be an issue. I got this cheap steel Thule -- Thule 990XT Doubletrack -- that clamps the wheels into their trays with plastic straps (like ski boots) and then uses a lockable padded hook on the crossbar to keep things from moving. Nothing clamps on the top of the wheels so fenders are not a problem. It works well and keeps our bikes well separated. When bikes are not loaded it can be lowered to access the rear hatch.